The present invention relates generally to the field of energy extraction from biomass. More particularly, it concerns energy extraction from plant biomass, such as plant fiber produced in grain, cane, and beet milling.
Biomass depleted of at least some carbohydrate, at least some oil, or both is used herein to refer to any material derived from a member of the plant kingdom and that has been physically separated from the plant and at least partially depleted (i.e., to less than about 40% d.s.b. total) of carbohydrate (sugars or starch) or oil (such as an oil-containing tissue, e.g., germ). An example of such biomass depleted of at least some carbohydrate, at least some oil, or both is fiber, defined herein as a material containing at least about 80 wt % d.s.b. of materials selected from the group consisting of non-starch polysaccharides, cellulose, hemicelluloses, dextrins, inulin, lignin, waxes, chitins, pectins, beta-glucans, oligosaccharides, and mixtures thereof. Other separated biomass includes, but is not limited to, such constituents as husk, hull, nutshells, leaves, stems, trunks, stalks, branches, or roots, among others. Biomass can be burned to provide energy; depending on the plant source, the biomass's composition, and its water content, it typically has a Higher Heating Value (HHV) of about 7000-10,000 BTU/lb (dry basis). Other plant materials, such as corn germ, can have HHVs as high as 15,000 BTU/lb (dry basis). Therefore, theoretically, biomass depleted of at least some carbohydrate, at least some oil, or both could be burned to wholly or partially power industrial processes, such as a milling process which produces fiber as a coproduct.
However, biomass depleted of at least some carbohydrate, at least some oil, or both has generally not been used as an energy source for a number of reasons. First, most biomass depleted of at least some carbohydrate, at least some oil, or both contains relatively high levels of ash (inorganic ions), such as phosphorous, calcium, magnesium, sodium, and potassium. For example, typical corn kernel fiber contains about 4% d.s.b. ash, of which phosphorous, either in elemental form or in compounds, is the most common element (total ash containing about 40 wt % P2O5). Fiber ash generally has a relatively low fusion or melting point, meaning that at higher temperatures the ash is molten and will form slag on refractory or metallic surfaces of a furnace, boiler, or flue gas stack if the molten ash contacts these surfaces. Ash at temperatures below its melting point is generally in the form of small, irregularly shaped, solid particles.
Another concern regarding combustion of biomass depleted of at least some carbohydrate, at least some oil, or both is fuel-bound nitrogen. Biomass depleted of at least some carbohydrate, at least some oil, or both typically contains some residual proteins, which contain nitrogen. Combustion in air or under another oxygenated atmosphere of a fuel containing nitrogen will lead to formation of nitrogen oxides (NOx). NOx emissions are generally discouraged under the regulatory climates prevailing in the developed world. NOx emissions can be at least partially reduced by good combustion practice. Further reduction of NOx can be achieved by scrubbing flue gas by various known means, but these involve further expenses for equipment, maintenance, and operations.
Therefore, it would be desirable to have a method for extracting energy from biomass depleted of at least some carbohydrate, at least some oil, or both that has a minimal risk of slagging and relatively low NOx emissions.